[14644-71-4]  · AlClH2  · Monochloroalane  · (MW 64.45)

(reducing agent for a variety of functional groups1)

Alternate Name: aluminum chloride hydride.

Physical Data: the reagent is generated and used in situ. See Aluminum Chloride and Lithium Aluminum Hydride.

Solubility: sol ether.

Preparative Methods: (a) A solution of 0.57 g (15 mmol) of LiAlH4 in 15 mL of dry ether is mixed with a solution of 1.99 g (15 mmol) of AlCl3 in 15 mL of dry ether at -5 °C. The resulting mixed hydride solution is stirred at room temperature for 15 min.2 (b) To a precooled (0-5 °C) 20 mL of ether are added ground AlCl3 (400 mg, 3.00 mmol) and LiAlH4 (114 mg, 3.00 mmol), and the suspension is heated under reflux for 30 min.3 This procedure provides AlH2Cl as a suspension in ether.

Analysis of Reagent Purity: Metal Hydrides Technical Bulletin No. 401 describes an apparatus and methodology for assay by means of hydrogen evolution. See also Rickborn and Quartucci.4 The IR spectrum of AlH2Cl shows a strong Al-H stretching absorption band at 1852 cm-1.5

Handling, Storage, and Precautions: solutions or suspensions of AlH2Cl in ether are highly flammable and must be stored in the absence of moisture. Tetrahydrofuran is cleaved by AlH2Cl to yield the corresponding aluminum alkoxide and is thus not a suitable solvent.

General Discussion.

This reagent readily reacts with aldehydes, ketones, esters, and epoxides to give alcohols, and also with amides, nitriles, and azides to give amines. Unlike Lithium Aluminum Hydride, this reagent does not reduce nitro groups (eq 1).6 Simple alkyl halides and tosylates are not readily reduced (eq 2).7 Optically active azetidin-2-ones are reduced to the corresponding azetidines without cleavage of the b-lactam ring or loss of enantiomeric purity (eqs 3 and 4).3

Aluminum chloride hydride is a milder reducing agent than LiAlH4, which acts as a Lewis acid as well as a hydride donor. For example, epoxides are reduced to the corresponding alcohols, where the ring cleavage takes place at the more substituted carbon (eqs 5 and 6).8 These results clearly show the strong Lewis acidity of this reagent.

Reduction of cyclopropene esters proceeds with carbon-carbon bond migration, giving cyclobutenes in good yield (eq 7).9 Reaction of 6b-methoxy-3a,5-cyclocholestane with AlH2Cl gives 3a,5-cyclo-5a-cholestane as the major product (eq 8).10

This reagent has a greater tendency to bring about hydrogenolysis than LiAlH4. Conjugated ketones and esters are frequently reduced to hydrocarbons (eqs 9-11).11

Aluminum chloride hydride is a convenient reagent for reductive cleavage of acetals to the corresponding ethers (eq 12),1 and hemithioacetals to thioethers (eq 13).12 Cyclic acetals are converted into hydroxy ethers (eqs 14 and 15).13,14 The regioselectivity of the reaction is governed by the electronic and steric effects of the substituents of the acetals (eq 16).15

Enamines are reduced to alkenes and amines in ether at reflux for 5-24 h, as exemplified in eq 17.16

1. (a) Eliel, E. L. Record Chem. Prog. 1961, 22, 129. (b) FF 1967, 1, 595.
2. Davis, H. A.; Brown, R. K. CJC 1971, 49, 2166.
3. (a) Yamashita, M.; Ojima, I. JACS 1983, 105, 6339. (b) Ojima, I.; Zhao, M.; Yamato, T.; Nakahashi, K. JOC 1991, 56, 5263.
4. Rickborn, B.; Quartucci, J. JOC 1964, 29, 3185.
5. Ashby, E. C.; Prather, J. JACS, 1966, 88, 729.
6. Nystrom, R. F. JACS 1955, 77, 2544.
7. Nystrom, R. F. JACS 1959, 81, 610.
8. (a) Eliel, E. L.; Delmonte, D. W. JACS 1958, 80, 1744. (b) Eliel, E. L.; Rerick, M. N. JACS 1960, 82, 1362. (c) Fristad, W. E.; Baily, T. R.; Paquette, L. A. JOC 1978, 43, 1620.
9. Gensler, W. J.; Langone, J. J.; Floyd, M. B. JACS 1971, 93, 3828.
10. Romeo, A.; Paradisi, M. P. JOC 1972, 37, 46.
11. (a) Cava, M. P.; Narasimhan, K. JOC 1969, 34, 3641. (b) Bridgeman, J. E.; Jones, E. R. H.; Meakins, G. D.; Wicha, J. CC 1967, 898. (c) Wigfield, D. C.; Taymaz, K. TL 1973, 4841.
12. Eliel, E. L.; Badding, V. JACS 1959, 81, 6087.
13. Clasper, P.; Brown, R. K. JOC 1972, 37, 3346.
14. Davis, H. A.; Brown, R. K. CJC 1973, 51, 361.
15. (a) Loewen, P. C.; Brown, R. K. CJC 1972, 50, 3639. (b) Loewen, P. C.; Makhubu, L. P.; Brown, R. K. CJC 1972, 50, 1502. (c) Diner, U. E.; Davis, H. A.; Brown, R. K. CJC 1967, 45, 207. (d) Jonas, J.; Breinek, P. CCC 1976, 41, 1188.
16. Lewis, J. W.; Lynch, P. P. Proc. Chem. Soc. 1963, 19.

Chung-Ying Tsai & Iwao Ojima

State University of New York at Stony Brook, NY, USA

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